Industrial users and residential and office facilities having large physical plants frequently, typically require both electric and fluid-based power for economical operation of various machines and heating and cooling systems. Usually the fluid-based power is high pressure steam, however, low pressure steam and high pressure water are also useful. Because high pressure steam is most often desired, the fluid-based power will be alternatively referred to below as "steam power" or "fluid-based power", as is appropriate. Depending upon prevailing economic conditions, users obtain the needed power according to various means. In recent years, as a result of increases in energy cost, many users have installed power generating facilities of their own to supplement or replace the power formerly obtained from municipal public utilities. Typically, the users' systems consist of a prime mover engine that produces shaft work and heat in the form of hot exhaust gases and hot cooling jacket fluid. The engines are usually internal combustion engines such as gas-fired engines or diesel engines, however, they may also be turbine engines. The shaft work runs an electric generator that produces electric power.
The fuel to fire the main engine is relatively expensive. The efficiency of these engines is relatively low, typically between 30 and 40 percent. By this is meant that the engine converts only 30 to 40 percent of the heat energy produced by the chemical reaction of burning the fuel into useful shaft work. Most of the missing heat energy is converted into heat in the form of high temperature exhaust, and also the heat which raises the temperature of the physical body of the engine. In most engines, the heating of the body is minimized by heat transfer to a cooling medium known as the "cooling jacket fluid." There are several types of cooling jacket systems. In some cases, a fluid, usually water, flows through conduits in the cooling jacket and is maintained at a flow rate and pressure such that it does not boil. This fluid, after leaving the engine in liquid form, may be rapidly de-pressurized in a flash tank to produce at least some vapor. When water is used as the cooling fluid, this type of cooling is known as "flash steam" cooling. In other cases, pressure and flow rates in the cooling jacket are established so that the cooling jacket fluid boils as a result of heat transfer from the engine. This latter type of cooling system is known as an "ebullient" system.
As will be described below, the cooling jacket fluid is used by the described apparatus and method to store and transfer energy from one segment of the apparatus to another. Usually, the cooling fluid is water, however, other fluids are possible. As the fluid is circulated through different parts of the apparatus, it is transformed from a liquid phase, to a mixed liquid and vapor phase, to a vapor phase and back to a liquid phase. Therefore, it is helpful to refer to the fluid as an "energy transfer fluid" when the location and state of the fluid need not be specified. This terminology will be used in the claims and when appropriate, in the following description. However, because the most common energy transfer fluid is water, the terminology for water, e.g. "steam" "water" "water vapor", will be used in the examples to identify the specific status of the energy transfer fluid.
Water emerging from the jacket after cooling an engine typically is a two-phase mixture of vapor and liquid including steam at a relatively low pressure (for example, 15 psig). Neither the liquid nor the vapor is readily useable in this state. Often, however, the user requires either high pressure steam or high pressure, high temperature water.
Various means to upgrade the cooling jacket steam are known. For example, U.S. Pat. No. 3,962,873, to Davis, discloses taking the relatively low pressure steam generated in the cooling jacket and compressing that steam to a high pressure using a compressor driven by the shaft work of the main engine. Further, the hot exhaust gases are used to heat pressurized water so that the water becomes high pressure steam. The user may take advantage of these two constituents of high pressure steam.
Various types of compressors are known for compressing the lower pressure steam to high pressure steam, for instance screw compressors, piston compressors, centrifugal compressors, etc.
Depending upon the type of compressor used to compress the lower pressure stem to high pressure steam, another problem often arises during operation of the prime mover engine at less than its design point load. This problem arises with the use of positive displacement compressor devices. By "positive displacement" compressor devices is meant a device that, at a given operating speed, attempts to pass a constant volume flow rate through its compression path. For instance, a screw compressor with a main rotor rotating at X revolutions per second will attempt to draw Y cubic feet of steam per second through its operating cavity. When the engine operates at less than design point load it provides a smaller amount, by volume, of cooling jacket steam to the compressor. Consequently, if no accommodations are made, the normal volume flow rate of steam presented to the compressor will be less than the amount the compressor attempts to draw.
Typically, the main shaft of the engine drives the compressor. Even at off design point the main shaft rotates at the same speed; only the torque of the generator is reduced. The compressor will continue to operate at its design point speed, and continue to attempt to draw more volume into its cavity than the system readily presents. As a consequence, a suction will arise and it is possible that the compressor will draw down the operating pressure of the entire engine system. This draw-down may ultimately result in elevated engine operating temperatures. As a result of the generation of vapor bubbles inside the engine block this condition could damage the engine and cause a shutdown of the entire system.
It is known to augment the mass flow rate at the input of the compressor by drawing compressed steam from the output of the compressor and recirculating this steam, after processing, to the input of the compressor, thereby bringing the mass flow rate up toward the design rate. This solution has the drawback that the high pressure steam must first be brought to a reduced pressure. Further, during the compression, work has been done on the steam, which work is also manifest as an elevated temperature. Thus, the steam must be cooled before reintroduction to the compressor. Otherwise, its temperature might rise to undesireable levels in the compressor. Typically the heat exchanged is conducted to the atmosphere, with a consequent loss of heat energy. This is undesireable due to the fact that energy was expended to raise the steam to that temperature, and thus energy is being thrown away.
In connection with private power generation systems, it is frequently desirable to be able to vary the amount of high pressure steam generated and the amount of electrical power generated. For instance, a typical engine operating at design point generates a certain amount of electrical power. According to the methods discussed above, the heat normally lost due to the exhaust gases and the cooling jacket steam is converted into high pressure steam. However, in some cases it is desirable to convert these two forms of heat into additional electric power.
This flexibility serves several purposes. The most obvious, of course, is that during different times of the year, an industry may have different needs for its production of goods, and utilization of machinery. Thus, for instance, a company may manufacture a certain part during the summer months, which part requires the use of many electric-power machines. During the same time, it may not be necessary to use high pressure steam, either due to the high ambient temperature or other considerations. In this caes, it would be beneficial to be able to convert the heat of the exhaust gases and the cooling jacket system to electrical power, rather than a high pressure steam.
Power utility rate structures also provide incentive for having flexible capability with respect to electric power generation. Power utilities generally charge an overall rate based on the maximum load required by the user. If the maximum load is above a certain plateau, then all of the power costs more, whether the maximum load is exceeded at the time or not. In some instances, the utilities audit the user's power demands for a short period of time during the course of the year. Thus, during the audit period it is obviously to the user's advantage to reduce its peak load to below the somewhat arbitrary plateau. There may also be more than one plateau. Thus, if the user can provide a relatively small amount of electric power on its own, especially during the period of energy audit, it stands to significantly reduce its energy bill.
U.S. Pat. No. 3,350,876, to Johnson issued Nov. 7, 1967 discloses an apparatus by which water from a cooling jacket is heated by the exhaust gases to a high temperature (and a high pressure), and is thereafter expanded in a turbine expander to generate shaft work. In this case, the turbine is attached to the main engine shaft and thus contributes to the power delivered to the electric generator. A drawback of this method is that the turbine may be used beneficially only to expand high pressure steam which leaves the turbine expander as dry steam. This is because themechanical elements of the turbine are relatively delicate and finely machined. If the expanding steam were permitted to achieve a partially liquid state, the turbine would be subject to mechanical errosion. This degradation is due to the constant impinging of water particles upon delicate metal portions. The degradation is similar to that caused by water droplets dripping upon or running across a rock in a mountain stream. Further, the turbine would be subject to corrosion caused by the deposition of liquid upon the turbine elements.
As has been mentioned above, a power generation system is a complicated system that typically operates under relatively low efficiencies. Therefore, every increase in efficiency, even though apparently small in magnitude, has the potential for contributing significantly to the improved operation of the power system. Therefore, it is desireable to combine all of the operating improvements and conveniences mentioned above, into an integrated system that utilizes a minimum number of elements and that requires minimum maintenance.
Therefore, the several objects of the invention disclosed herein, as will be explained in the detailed discussion that follows, or that may be readily apparent to one ordinarily skilled in the art are as follows: To provide an apparatus and a method of power generation that produces both electric and steam poewr and that takes advantage of substantially all of the heat generated by the prime mover engine to produce either steam power or electric power; to provide an electric and steam power cogeneration system that produces a significant amount of high pressure steam and that may be operated at off design point load; to provide a steam power and electric power cogeneration system that permits a flexible choise of enhanced steam of electric power or both utilizing the same mechanical elements.